Raman scattering from surface optic (SO) phonons has been observed and identified in cylindrical and rectangular cross-section nanowires with lateral dimensions in the range 20-50 nm and lengths of ~ 10 microns. The position of the SO band is found to depend on both the shape of the wire cross section (i. e., circular or rectangular) and on the dielectric constant of the external medium. The position of the SO band in GaP and ZnS nanowires is in good agreement with a dielectric continuum model that takes the shape of the wire cross section into account. We propose that the symmetry breaking mechanism which activates the Raman sacttering of the SO phonon in our nanowires is a quasi-periodic modulation of the cross-sectional area along the nanowire axis. We suggest this modulation stems from an interesting growth instability associated with vapor-liquid-solid (VLS) growth.